Apparatus for fire detection and alarm



April 12, 1966 E. A. SACK APPARATUS FOR FIRE DETECTION AND ALARM Filed May 9, 1962 ECTRIC GENERATOR THERMOEL THERMOELECTRIC GENERATOR AT=75C AT=56C l.O AMPERES Fig.3

INVENTOR Edgar A. Sock ATTORN United States Patent 3 246,311 APPARATUS FUR Fllitl. DETECTION AND ALAN i Edgar A. Sack, Penn Hills, Pa, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed May 9, 1962, Ser. No. 193,427 3 Claims. (Cl. 340228) The present invention relates generally to apparatus for fire detection and alarm and more particularly relates to fire alarm apparatus utilizing the energy released by the fire itself.

Late discovery of the outbreak of fires is a related factor in loss of life and property damage. Fire alarm systems with detectors placed strategically in any structure greatly reduces the loss of life and property due to a fire. Industrial fire alarm systems are not suitable for the home or small establishments due to their cost and difficulty of installation in existing structures. Commercial fire alarms which rely on connection to the power mains lack reliability in that the power is often the first thing to fail in a fire or its failure may be the cause of the fire. Conventional low cost fire alarms presently available lack reliability.

The popular battery-operated fire detector works very well in the home except that the home owner forgets to replace the battery at appropriate intervals with the result that the alarm is not operative when it is needed.

The ideal fire alarm device is one which anyone can install, which is free from dependence on external power, has infinite standby life without maintenance, and provides an alarm for all to hear when a fire starts.

The present invention provides an ultrareliable fire alarm through the use of solid state devices. Briefly, a thermoelectric power supply is adapted to provide a heatproduced voltage to an electroaudio transducer. Such apparatus may include a tunnel diode to sustain oscillations. The oscillation frequency can be made to be in the audible range by proper selection of parameter values.

Accordingly, an object of the present invention is to provide an ultrareliable fire alarm system.

Another object of the present invention is to provide fire alarm apparatus independent of external electrical power.

Another object of the present invention is to provide fire alarm apparatus which is easily installed, free of maintenance, exhibiting long standby life and providing an audio alarm upon occurrence of fire.

Another object of the present invention is to provide fire alarm apparatus in which electrical oscillations are established in response to heat from a fire with such oscillations driving an electroaudio transducer.

Further objects and advantages of the present invention will be readily apparent from the following detailed description taken in conjunction with the drawing, in which:

FIGURE 1 is an electrical schematic diagram of an illustrative embodiment of the present invention;

FIG. 2 is an electrical schematic diagram of an alternate illustrative embodiment of the present invention;

FIG. 3 is a characteristic curve of a device utilized in the illustrative embodiment shown in FIG. 1;

FIG. 4 is a characteristic curve of a device utilized in the alternate illustrative embodiment shown in FIG. 2;

FIG. 5 illustrates a thermal geometrical form for the present invention;

FIG. 6 illustrates an alternate thermal-geometrical form for the present invention; and

FIG. 7 illustrates another alternate thermal-geometrical form for the present invention.

FIG. 1 best illustrates the utilization of the energy released by the fire itself to sound the alarm. A thermoelectric geuerator 2 is connected to provide its electrical 3 ,246 ,31 1 Patented Apr. 12, 1966 output across a vibrating arm electroaudio transducer 4. The electroaudio transducer 4 includes a vibrating arm 6 and inductance 8 adapted to drive a horn lit) or similar alarm means.

For example in one successful installation, a thermoelectric generator 2 having fifty couples in series having an internal resistance of approximately 0.7 ohms was utilized. The electroaudio transducer 4 which was used had a pull-in current requirement of 0.6 amperes at a pull-in potential of 0.3 volts. The average running current was substantially 70% of the pull-in current and the input power was approximately milliwatts. From the electrical characteristics of the aforementioned fifty couple thermoelectric power supply as illustrated in FIG. 3, it can be seen that a thermal differential across the thermoelectric generator 2 of approximately 45 C. will provide the necessary pull-in current for the electroaudio transducer 4.

FIG. 2 illustrates a fire alarm circuit eliminating the vibrating arm 6. A thermoelectric generator 12 is connected in series circuit combination with an electroaudio transducer 14, a tunnel diode 16 and a resistance element 18.

The tunnel diode 16 has an I-V characteristic curve as shown in FIG. 4. The device freely allows conduction for increases in potential in the reverse direction. In the forward direction of voltage across the device, the current therethrough increases to a sharp maximum I on a portion of the characteristic curve to be referred to as the low voltage range. Further increase in the voltage across the device results in the negative resistance portion of the characteristic curve wherein the current through the diode drops to a deep and broad minimum, referred to as the valley current I Still further increase in the voltage across the diode causes the current to increase again on a portion of the characteristic curve to be referred to as the high voltage range, to a maximum current 1 and a maximum voltage V across the device as determined by the circuit parameters of the series circuit combination. For purposes of this specification, the term, tunnel diode, is herein meant to include all devices exhibiting the aforementioned characteristics.

The electro acoustic transducer 14 has a coil 15 of inductance L. The equivalent resistance of the series circuit combination excluding the tunnel diode has a magnitude R, represented by the positive resistance ele' meat 18. In operation upon occurrence of fire the temperature differential across the thermoelectric generator 12 increases and the voltage generated thereby will likewise increases. As the voltage increases, the current also increases through the series circuit combination and hence through the tunnel diode 2 until the current peak I of the tunnel diode 16 is exceeded. The tunnel diode switches from the low voltage range of its characteristic curve to the high voltage range. As a result, the resistance of the tunnel diode 16 to current flow increases and the current through the series circuit combination starts diminishing. As the current decreases, the tunnel diode 16 will drop down to the low voltage range of its characteristic curve with the result that the voltage thereacr-oss will also be greatly reduced. Upon occurrence of a small voltage across the tunnel diode 16, the voltage across the inductance coil 15 will be large and the current once again builds up until 1 is exceeded. In such a manner the current and voltage relation of the tunnel diode 16 oscillates tracing approximately the path ABCD indicated by the arrows in the characteristic curve of FIG. 4- during each cycle of oscillation.

When the voltage from the thermoelectric generator 12 is small or zero, no oscillation is possible in the circuit. Once the voltage becomes large enough to drive the tunnel diode 16 into the negative resistance region,

oscillation must start. The circuit parameters are chosen to result in a strong relaxation oscillation at an audio frequency. Referring to FIG. 2 oscillation will occur if two conditions are satisfied. First,

R Tiro where R is the equivalent resistance of the series circuit excluding the tunnel diode and Rn is the magnitude of the tunnel diode average negative resistance. In addition where L is the inductance of the coil 15 and C is the parasitic shunt capacitance of the tunnel diode 16. It can be shown that a strong relaxation oscillation is obtained if the inequality is satisfied.

The parameters must also be chosen so that the relaxation oscillation occurs at an audio rate. Because of the highly nonlinear behavior of the circuit in the relaxation made, the oscillation period cannot be expressed in simple terms. However, the oscillation period is largely controlled by the rate at which energy may be stored or removed from coil L during the time the tunnel diode curve is traversed from A to B and C to D in FIG- URE 4. This rate is given approximately by the time constants L/R and L/R where R and R are the circuit resistances when the tunnel diode is in these regions.

It is readiiy apparent that in as much as a violent relaxation oscillation occurs in the circuit of FIG. 2 as a temperature gradient is established across the thermoelectric generator 12, a truely static fire alarm apparatus is provided without mechanical contacts thereby increasing overall system reliability.

FIGS. 5, 6 and 7 suggest different thermalgeometrical forms for the ultrareliable fire alarm apparatus in accordance with the present invention. Similar devices have been designated by like reference characters for the sake of clarity. Any configuration should be such that the system may be mounted by the average home owner through a simple process of perhaps drilling a hole through a partition between two rooms.

Referring specifically to FIG. 5, small medallions Zii and 22 disposed on opposite sides of a wall partition 24 provide mounting bracket means as well as a temperature difierential path 26 across a thermoelectric generator 28. A thermal path 2.6 conducts heat from the room on one side of the partition 24 to one end of the thermoelectric generator 28. The opposite end of the generator 28 is exposed to the temperature of the opposite side of the partition through the medallion 22. The temperature differential across the thermoelectric generator 28 is provided by the variance in temperature thereacross as indicated by 91 and 6'2. The thermal path 26 conducts heat to the generator 28 in an efiicient manner to provide maximum temperature difierential thereacross. An electroaudio transducer and tunnel diode electrically connected in accordance with FIG. 2 or merely the electroaudio transducer in accordance with FIG. 1 may be mounted within the medallion 22.

An alternate construction as shown in FIG. 6 eliminates the thermal path 28 with the medallions 20 and 22 providing mounting brackets for stretching a thermoelectric generator 30 across the width of the partition 24.

The thermal-geometrical forms indicated in FIGS. 5 and 6 are only by way of illustration. Other manners of mounting may be had other than through a partition between two rooms. The thermal gradient can also be established across the thermocouple by providing a thermal mass behind the fire alarm as shown in FIG. 7. Medallion 22 which collects heat from the fire is in thermal contact with one side of thermocouple 23. The other side of the couple is in contact with thermal block 31 which may be mounted against the wall. Because the medallion 22 which has a lower thermal mass heats more rapidly than thermal block 31 which has a much larger mass, a significant thermal gradient is established across the thermocouple 28 thus providing the electrical power needed to operate the fire alarm. in some installations the alarm may be mounted through the wall behind a picture, through a door, from the room side of a wall into a partition space, and other likely locations. The only requisite is that the alarm be mounted in such a manner that a thermal gradient will exist across the ther moelectric generator when fire starts. In the ideal situation, the alarm is sufiiciently inexpensive that the home owner can mount one or more near the ceiling in every room, particularly at the top of stairwells or near obviously hazardous locations.

While the present invention has been described with a degree of particularity for the purposes of illustration, it is to be understood that all modifications, alterations and equivalents within the spirit and scope of the present invention are herein meant to be included. For example, it will occur to those skilled in the art that greater reliability may be achieved through the use of a piezoelectric instead of a magnetic transducer. Some parts of the circuit illustrated may be formed on a single block, or additional components, such as a capacitor to shunt the thermoelectric generator, may be added for improved operation.

I claim as my invention:

1. Fire alarm apparatus responsive to ambient temperature comprising, in combination; an elcctroaudio transducer having an inductance element; a tunnel diode; temperature gradient responsive power supply means and a pair of heat sinks disposed at a distance apart, said power supply means eing disposed between said pair of heat sinks to sense the ambient temperature differential therebetween; a series circuit combination including said inductance element, said tunnel diode and said power supply means; said circuit combination having an effective positive resistance magnitude greater than the negative resistance magnitude of said tunnel diode so that said circuit combination will oscillate to activate said transducer when a predetermined temperature differential exists between said heat sinks.

2. Apparatus for fire detection responsive to ambient temperature comprising, in combination; thermoresponsive power means; mounting brackets for disposing said thermoresponsive power means between two locations to sense the ambient temperature differential therebetween; a semiconductor device having a characteristic curve a portion of which exhibits a negative resistance; said power means biasing said semiconductor device to the negative portion of its characteristic curve upon the temperature difierential across said power means exceeding a predetermined magnitude; electroaudio transducer means including an inductance coil; circuit means for connecting said inductance coil, said semiconductor device, and said power means in series circuit combination; the magnitude of the effective positive resistance of said circuit means being sufi'lciently greater than the magnitude of the negative resistance presented by said semiconductor device to force said series circuit combination into relaxation oscillation to activate said electroaudio transducer means upon the voltage signal from said thermoelectric generator reaching a predetermined magnitude.

3. Fire alarm apparatus responsive to ambient temperature comprising, in combination; an electroaudio transducer having an inductance element; a tunnel diode; and temperature gradient responsive power supply means,

mounting means for disposing said power supply means between two locations to sense the ambient temperature differential therebetween and provide an output voltage in response there-to; a series circuit combination including said inductance element, said tunnel diode and said power supply means; the parameters of said circuit combination being chosen so that said circuit combination will oscillate to activate said transducer when a predetermined temperature differential exists between said two locations, said parameters being where R is the equivalent resistance of said circuit combination excluding the tunnel diode and Rn is the magnitude of the tunnel diode average negative resistance, and

L CRlRnI where L is the inductance of said element and C is the parasitic shunt capacitance of said diode.

References Cited by the Examiner UNITED STATES PATENTS Chow, W. F. et aL: Tunnel Diode Circuit Aspects and Applications, AIEE Conference Paper 60-297, January 1960, pp. 3, 4, 9, 18, 19 and 20.

NEIL C. READ, Primary Examiner.

R. M. ANGUS, Assistant Examiner. 

1. FIRE ALARM APPARATUS RESPONSIVE TO AMBIENT TEMPERATURE COMPRISING, IN COMBINATION; AN ELECTROAUDIO TRANSDUCER HAVING AN INDUCTANCE ELEMENT; A TUNNEL DIODE; TEMPERATURE GRADIENT RESPONSIVE POWER SUPPLY MEANS AND A PAIR OF HEAT SINKS DISPOSED AT A DISTANCE APART, SAID POWER SUPPLY MEANS BEING DISPOSED BETWEEN SAID PAIR OF HEAT SINKS TO SENSE THE AMBIENT TEMPERATURE DIFFERENTIAL THEREBETWEEN; A SERIES OF CIRCUIT COMBINATION INCLUDING SAID INDUCTANCE ELEMENT, SAID TUNNEL DIODE AND SAID POWER SUPPLY MEANS; SAID CIRCUIT COMBINATION HAVING AN EFFECTIVE POSITIVE RESISTANCE MAGNITUDE GREATER THAN THE NEGATIVE RESISTANCE MAGNITUDE OF SAID TUNNEL DIODE SO THAT SAID CIRCUIT COMBINATION WILL OSCILLATE TO ACTIVATE SAID TRANSDUCER WHEN A PREDETERMINED TEMPERATURE DIFFERENTIAL EXISTS BETWEEN SAID HEAT SINKS. 